Stents are typically used as adjuncts to percutaneous transluminal balloon angioplasty procedures, in the treatment of occluded or partially occluded arteries and other blood vessels. As an example of a balloon angioplasty procedure, a guiding catheter or sheath is percutaneously introduced into the cardiovascular system of a patient through the femoral arteries and advanced through the vasculature until the distal end of the guiding catheter is positioned at a point proximal to the lesion site. A guidewire and a dilatation catheter having a balloon on the distal end are introduced through the guiding catheter with the guidewire sliding within the dilatation catheter. The guidewire is first advanced out of the guiding catheter into the patient's vasculature and is directed across the arterial lesion. The dilatation catheter is subsequently advanced over the previously advanced guidewire until the dilatation balloon is properly positioned across the arterial lesion. Once in position across the lesion, the expandable balloon is inflated to a predetermined size with a radiopaque liquid at relatively high pressure to radially compress the atherosclerotic plaque of the lesion against the inside of the artery wall and thereby dilate the lumen of the artery. The balloon is then deflated to a small profile so that the dilatation catheter can be withdrawn from the patient's vasculature and blood flow resumed through the dilated artery.
Balloon angioplasty sometimes results in short or long term failure (restenosis). That is, vessels may abruptly close shortly after the procedure or restenosis may occur gradually over a period of months thereafter. To counter restenosis following angioplasty, implantable intraluminal prostheses, commonly referred to as stents, are used to achieve long term vessel patency. A stent functions as scaffolding to structurally support the vessel wall and thereby maintain luminal patency, and are transported to a lesion site by means of a delivery catheter.
Types of stents may include balloon expandable stents, spring-like, self-expandable stents, and thermally expandable stents. Balloon expandable stents are delivered by a dilitation catheter and are plastically deformed by an expandable member, such as an inflation balloon, from a small initial diameter to a larger expanded diameter. Self-expanding stents are formed as spring elements which are radially compressible about a delivery catheter. A compressed self-expanding stent is typically held in the compressed state by a delivery sheath. Upon delivery to a lesion site, the delivery sheath is retracted allowing the stent to expand. Thermally expandable stents are formed from shape memory alloys which have the ability to expand from a small initial diameter to a second larger diameter upon the application of heat to the alloy.
Polymeric materials are increasingly being utilized in intraluminal prostheses, such as stents, as well as in other types of medical devices used within the bodies of subjects. Polymeric materials conventionally utilized in the medical device industry for implantation within the bodies of subjects include, but are not limited to polyurethanes, polyolefins (e.g., polyethylene and polypropylene), poly(meth)acrylates, polyesters (e.g., polyethyleneterephthalate), polyamides, polyvinyl resins, silicon resins (e.g., silicone rubbers and polysiloxanes), polycarbonates, polyfluorocarbon resins, synthetic resins, and polystyrene.
Many conventional polymeric materials contain a range of additives (e.g., plasticizers, antioxidants, UV stabilizers, etc.) as well as a host of contaminants (e.g., residual monomer, oligomers, solvent residues, catalysts, initiators, etc.). For example, casting solvents such as dimethyl sulfoxide (DMSO), chloro-organics, aromatics, tetrahydrofuran (THF), etc. are conventionally utilized in stent production. Moreover, various toxic organic solvents and plasticizers are conventionally used to impregnate the polymeric material of implantable devices, such as intraluminal prostheses, with pharmacological agents. Trace quantities of these materials may remain in the polymeric materials during fabrication of these devices and patients receiving these devices, or pharmacological agents eluted therefrom, may be exposed to these potentially toxic materials, particularly when the implantable device erodes.
As such, it is desirable to purify polymeric materials utilized in medical devices, such as intraluminal prostheses, in order to remove solvents and other potentially toxic materials and to enhance the biocompatibility of the polymeric material. Unfortunately, conventional purification methods may involve applying heat to the polymeric material. The addition of heat may alter the physical characteristics of the polymeric material, thus negatively affecting the biocompatibility of the material.